Field effect transistors, as well as bipolar transistors, are widely used as important switches or amplifying elements. The transistor comprises: a semiconductor layer which forms a current path between a source electrode and a drain electrode; a gate electrode which controls the flow of the current by applying voltage; and an insulator layer which separates the gate electrode from the semiconductor layer. The characteristics of the field effect transistor are determined by characteristics of a semiconductor to be used. In particular, the carrier mobility and on/off value are important factors.
Inorganic materials such as amorphous silicon, polysilicon, etc. have been widely used as semiconductor materials. Inorganic semiconductor material typically represented by silicon is a single crystal of a single element, has a simple structure and is stable in physical properties. However, such silicon needs high temperature treatments so that it is difficult for a plastic base plate or film to be used as a substrate for field effect transistors. In addition, expensive manufacturing facilities are required because devices are produced under vacuum environment, being expensive.
Recently, instead of the inorganic semiconductors, organic field effect transistors using organic semiconductors are attracting attention, and studies on the organic semiconductors have been recently proceeded with great speed in terms of fundamental optoelectronics. These organic field effect transistors enable the production of light-weight, mechanically-flexible and large-area transistors by using organic semiconductors.
Organic semiconductors are classified into 3 types: a p-type semiconductor in which a positively charged hole has a role of transmitting electrical current; an n-type semiconductor in which a negatively charged free electron has a role of transmitting electrical current; and an ambipolar type semiconductor which has both p-type and n-type roles.
For example, Japanese Patent Publication 2007-266411A discloses a field effect transistor that uses a benzodifuranone-type organic compound as a semiconductor material. This field-effect transistor has n-type, p-type and ambipolar-type electrical characteristics.
A number of organic semiconductors having p-type characteristics are just starting developing in recent years, but the development of the p-type organic transistors is still insufficient. As for the n-type and ambipolor-type organic semiconductors, the number itself of the studies on their materials has been very small so far.
Desired are organic semiconductor materials, regardless of transistor type (n-type, p-type, or ambipolar type), having high carrier mobility and high on/off ratio, and also having excellent processability such that a solution of organic semiconductor material can be, coated or printed using an ink-jet printing technique. In particular, desired are the development of organic semiconductor materials from which a low-power-consumption complementary-type metal-oxide semiconductor (CMOS) circuit can be easily produced, because a transistor made of ambipolar-type organic semiconductor makes the transistor possible to have both p-type and n-type drive. (Evaluation and Application of Organic Transistor Materials II, CMC Publishing Co., LTD, 2008, pp. 81-94)
Organic semiconductor materials at present have little practicability due to its poor solubility, difficulty in synthesis, and insufficient carrier mobility, etc.